The present invention relates to a dot matrix line printer having the capability of eliminating slack in sheets of print paper.
FIG. 1 shows an arrangement of a prior art dot line printer. A hammer bank 12, accommodating a plurality of print hammers (not shown) therein, is reciprocated in the direction perpendicular to a sheet feeding direction. The print hammers are selectively fired during the reciprocal movement of the hammer bank 12 so as to make dot impressions on a continuous form print paper 1 through an ink ribbon 2. Those dot impressions collectively form a dot matrix character or symbol. A platen 13 is disposed rearwardly of the hammer bank 12 with a certain spacing therebetween. The print paper 1 is provided with uniformly-spaced perforations at its side margins, which are drivingly engaged by a pair of pin tractors 11 which entrain the print paper 1. The print paper 1 is moved incrementally past the space between the hammer bank 12 and the platen 13 while being guided by paper guide members 14. The pin tractors 11 and the platen 13 are synchronously driven by a paper feed motor 3 through timing belts 15, 16 and cooperatively feed the print paper 1 on a dot line basis. To prevent the print paper 1 from rippling on the platen at the time of paper feeding, there are provided an elongated friction plate 17 and a paper braking mechanism 18. The friction plate 17 is disposed closely to the peripheral surface of the platen. The paper braking mechanism 18 is disposed at a position beneath the platen 13.
When many blank lines are created at a time after printing several lines, such as the case for renewing a page, the print paper 1 tends to overrun due to an accelerated movement of the print paper 1, thereby yielding a slack of the print paper 1 at a portion between the pin tractor 11 and the paper braking mechanism 18. If the print paper 1 is further fed from this condition, the paper does not actually advance as much as it is fed due to the presence of the paper slack. If the printing is carried out in such a condition, the height of the printed characters is compressed or a line spacing becomes unequal.
A similar problem exists when the print paper 1 is loaded or the ink ribbon 2 is replaced. To facilitate loading of the print paper 1 or replacement of the ink ribbon 2, the spacing between the hammer bank 12 and the platen 11 is widened, with the result that the print paper 1 which has been applied with a tension by the friction plate 17 and the paper braking mechanism 18 becomes free from tensive force. When the spacing between the hammer bank 12 and the platen 13 is changed to the original state, the print paper 1 may be slackened.